Group iii nitride bulk crystals and their fabrication method

ABSTRACT

In one instance, the invention provides a bulk crystal of group III nitride having a thickness of more than 1 mm without cracking above the sides of a seed crystal. This bulk group III nitride crystal is expressed as Ga x1 Al y1 In 1-x1-y1 N (≦x1≦1, 0≦x1+y1≦1) and the seed crystal is expressed as Ga x2 Al y2 In 1-x2-y2 N (0≦x2≦1, 0≦x2+y2≦1). The bulk crystal of group III nitride can be grown in supercritical ammonia or a melt of group III metal using at least one seed crystal having basal planes of c-orientation and sidewalls of m-orientation. By exposing only c-planes and m-planes in this instance, cracks originating from the sides of the seed crystal are avoided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. App. No. 62/002,488 filed May23, 2014 and entitled “Group III Nitride Bulk Crystals and TheirFabrication Method,” inventors Tadao Hashimoto and Edward Letts, theentire content of which is incorporated by reference herein for allpurposes.

This application is also related to the PCT application filed on samedate as this application and entitled “Group III Nitride Bulk Crystalsand Their Fabrication Method,” inventors Tadao Hashimoto and EdwardLetts, the entire content of which is incorporated by reference hereinfor all purposes.

This application is also related to the following U.S. patentapplications:

PCT Utility Patent Application Serial No. US2005/024239, filed on Jul.8, 2005, by Kenji Fujito, Tadao Hashimoto and Shuji Nakamura, entitled“METHOD FOR GROWING GROUP III-NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIAUSING AN AUTOCLAVE,” attorneys' docket number 30794.0129-WO-01(2005-339-1);

U.S. Utility patent application Ser. No. 11/784,339, filed on Apr. 6,2007, by Tadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled“METHOD FOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS INSUPERCRITICAL AMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,”attorneys docket number 30794.179-US-U1 (2006-204), which applicationclaims the benefit under 35 U.S.C. Section 119(e) of U.S. ProvisionalPatent Application Serial No. 60/790,310, filed on Apr. 7, 2006, byTadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled “A METHODFOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS IN SUPERCRITICALAMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,” attorneysdocket number 30794.179-US-P1 (2006-204);

U.S. Utility patent application Ser. No. 60/973,602, filed on Sep. 19,2007, by Tadao Hashimoto and Shuji Nakamura, entitled “GALLIUM NITRIDEBULK CRYSTALS AND THEIR GROWTH METHOD,” attorneys docket number30794.244-US-P1 (2007-809-1);

U.S. Utility patent application Ser. No. 11/977,661, filed on Oct. 25,2007, by Tadao Hashimoto, entitled “METHOD FOR GROWING GROUP III-NITRIDECRYSTALS IN A MIXTURE OF SUPERCRITICAL AMMONIA AND NITROGEN, AND GROUPIII-NITRIDE CRYSTALS GROWN THEREBY,” attorneys docket number30794.253-US-U1 (2007-774-2);

U.S. Utility Patent Application Ser. No. 61/067,117, filed on Feb. 25,2008, by Tadao Hashimoto, Edward Letts, Masanori Ikari, entitled “METHODFOR PRODUCING GROUP III-NITRIDE WAFERS AND GROUP III-NITRIDE WAFERS,”attorneys docket number 62158-30002.00 or SIXPOI-003;

U.S. Utility Patent Application Ser. No. 61/058,900, filed on Jun. 4,2008, by Edward Letts, Tadao Hashimoto, Masanori Ikari, entitled“METHODS FOR PRODUCING IMPROVED CRYSTALLINITY GROUP III-NITRIDE CRYSTALSFROM INITIAL GROUP III-NITRIDE SEED BY AMMONOTHERMAL GROWTH,” attorneysdocket number 62158-30004.00 or SIXPOI-002;

U.S. Utility Patent Application Ser. No. 61/058,910, filed on Jun. 4,2008, by Tadao Hashimoto, Edward Letts, Masanori Ikari, entitled“HIGH-PRESSURE VESSEL FOR GROWING GROUP III NITRIDE CRYSTALS AND METHODOF GROWING GROUP III NITRIDE CRYSTALS USING HIGH-PRESSURE VESSEL ANDGROUP III NITRIDE CRYSTAL,” attorneys docket number 62158-30005.00 orSIXPOI-005;

U.S. Utility Patent Application Ser. No. 61/131,917, filed on Jun. 12,2008, by Tadao Hashimoto, Masanori Ikari, Edward Letts, entitled “METHODFOR TESTING III-NITRIDE WAFERS AND III-NITRIDE WAFERS WITH TEST DATA,”attorneys docket number 62158-30006.00 or SIXPOI-001;

U.S. Utility patent application Ser. No. 61/106,110, filed on Oct. 16,2008, by Tadao Hashimoto, Masanori Ikari, Edward Letts, entitled“REACTOR DESIGN FOR GROWING GROUP III NITRIDE CRYSTALS AND METHOD OFGROWING GROUP III NITRIDE CRYSTALS,” attorneys docket number SIXPOI-004;

U.S. Utility patent application Ser. No. 61/694,119, filed on Aug. 28,2012, by Tadao Hashimoto, Edward Letts, Sierra Hoff, entitled “GROUP IIINITRIDE WAFER AND PRODUCTION METHOD,” attorneys docket numberSIXPOI-015;

U.S. Utility Patent Application Ser. No. 61/705,540, filed on Sep. 25,2012, by Tadao Hashimoto, Edward Letts, Sierra Hoff, entitled “METHOD OFGROWING GROUP III NITRIDE CRYSTALS,” attorneys docket number SIXPOI-014;

which applications are incorporated by reference herein in theirentirety as if put forth in full below.

BACKGROUND

1. Field of the Invention

The invention relates to a bulk crystal of semiconductor material usedto produce semiconductor wafers for various devices includingoptoelectronic devices such as light emitting diodes (LEDs) and laserdiodes (LDs), and electronic devices such as transistors. Morespecifically, the invention provides a bulk crystal of group III nitridesuch as gallium nitride. The invention also provides various methods ofmaking these crystals.

2. Description of the Existing Technology

This document refers to several publications and patents as indicatedwith numbers within brackets, e.g., [x]. Following is a list of thesepublications and patents:

[1] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 6,656,615.

[2] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 7,132,730.

[3] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 7,160,388.

[4] K. Fujito, T. Hashimoto, S. Nakamura, International PatentApplication No. PCT/US2005/024239, WO07008198.

[5] T. Hashimoto, M. Saito, S. Nakamura, International PatentApplication No. PCT/US2007/008743, WO07117689. See also US20070234946,U.S. application Ser. No. 11/784,339 filed Apr. 6, 2007.

[6] D'Evelyn, U.S. Pat. No. 7,078,731.

Each of the references listed in this document is incorporated byreference in its entirety as if put forth in full herein, andparticularly with respect to their description of methods of making andusing group III nitride substrates.

Gallium nitride (GaN) and its related group III nitride alloys are thekey material for various optoelectronic and electronic devices such asLEDs, LDs, microwave power transistors, and solar-blind photo detectors.Currently LEDs are widely used in displays, indicators, generalilluminations, and LDs are used in data storage disk drives. However,the majority of these devices are grown epitaxially on heterogeneoussubstrates, such as sapphire and silicon carbide because GaN substratesare extremely expensive compared to these heteroepitaxial substrates.The heteroepitaxial growth of group III nitride causes highly defectedor even cracked films, which hinder the realization of high-end opticaland electronic devices, such as high-brightness LEDs for generallighting or high-power microwave transistors.

To solve fundamental problems caused by heteroepitaxy, it isindispensable to utilize crystalline group III nitride wafers slicedfrom bulk group III nitride crystal ingots. For the majority of devices,crystalline GaN wafers are favorable because it is relatively easy tocontrol the conductivity of the wafer and GaN wafer will provide thesmallest lattice/thermal mismatch with device layers. However, due tothe high melting point and high nitrogen vapor pressure at elevatedtemperature, it has been difficult to grow GaN crystal ingots.Currently, the majority of commercially available GaN substrates areproduced by a method called hydride vapor phase epitaxy (HVPE). HVPE isone of vapor phase methods, which has difficulty in reducing dislocationdensity less than 10⁵ cm⁻².

To obtain high-quality GaN substrates for which dislocation density isless than 10⁵ cm⁻², various growth methods such as ammonothermal growth,flux growth, high-temperature solution growth have been developed.Ammonothermal method grows group III nitride crystals in supercriticalammonia [1-6]. The flux method and the high-temperature solution growthuse a melt of group III metal.

Recently, high-quality GaN substrates having dislocation density lessthan 10⁵ cm⁻² can be obtained by ammonothermal growth. Since theammonothermal method can produce a true bulk crystal, one can grow oneor more thick crystals and slice them to produce GaN wafers. We havebeen developing bulk crystals of GaN by the ammonothermal method.However, we found it quite challenging to avoid cracking of bulkcrystals, especially when the total thickness of the bulk crystalexceeds 1 mm. We believe that the cracking problem in bulk group IIInitride is a universal problem for any bulk growth methods including theammonothermal method. Thus, this invention is intended to obtaincrack-free bulk group III nitride crystals using any bulk growth method,such as growth in supercritical ammonia or from a melt of group IIImetals.

SUMMARY OF THE INVENTION

In one instance, the invention provides a bulk crystal of group IIInitride having a thickness of more than 1 mm without cracking above thesides of a seed crystal. This bulk group III nitride crystal isexpressed as Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1) and theseed crystal is expressed as Ga_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1,0≦x2+y2≦1). The bulk crystal of group III nitride can be grown insupercritical ammonia or a melt of group III metal using at least oneseed crystal having basal planes of one orientation and all sidewalls ofone or more other orientations that are slow-growing compared to thebasal plane. For instance, the group III nitride seed crystal may havec-orientation basal planes and all sidewalls may be prismatic crystalfaces with m-plane orientation. By exposing only c-planes and m-planes,cracks originating from the sides of the seed crystal are avoided.

The invention also provides a new seed crystal that can be used to growbulk group III nitride. The seed crystal is expressed asGa_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1, 0≦x2+y2≦1) and has one or more ofthe following traits in any combination: (A) the seed crystal may have(1) exposed basal planes on which most growth occurs and (2) exposedsidewalls that grow slowly compared to the basal planes; (B) each of thesidewalls of a seed crystal may grow at a rate that is less than 20 μmper day, while optionally a face essentially parallel to a basal planemay grow at a rate that is greater than 20 μm per day; (C) all sidewallsmay grow at a rate that is less than 20% of the rate at which a facegrows; (D) the basal planes of the seed crystal may be c-planes, and thesidewalls of the seed crystal may all be m-plane; (E) the seed crystalmay be formed by hydride vapor-phase epitaxy (HVPE), molecular beamepitaxy (MBE), metal organic vapor-phase epitaxy (MOVPE), ammonothermalgrowth, or other method; (F) the seed crystal may have a hexagonalshape, a triangular shape, or a shape of a rhombus having no rightangles.

In addition, the invention provides new methods of making a seed crystalas well as new methods of making a bulk crystal of group III nitride aswell as wafers, optical devices, and semiconductor devices. Thefollowing methods may be used alone or in any combination. A seedcrystal as described above can be made by growing group III nitridehaving the formula Ga_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1, 0≦x2+y2≦1) byvapor-phase epitaxy (HVPE), molecular beam epitaxy (MBE), metal organicvapor-phase epitaxy (MOVPE), or ammonothermal growth, for example, andremoving a portion of the group III nitride to form a seed that has afirst face, a second face, and a plurality of side-walls extendingbetween the faces at the faces' periphery, with all of the sidewallsbeing slow-growing as compared to growth on the first face. Thesidewalls may each have a length that is within +/−10% of the averagelength of the sidewalls. A seed crystal may be made by forming a rawseed crystal from group III nitride using HVPE, MBE, or MOVPE, andshaping this raw seed crystal to have slow-growing sidewalls and afaster-growing face. The method may further comprise placing twoessentially identical group III nitride crystals together such thatGa-polar faces touch one another to form a new seed prior to making abulk crystal. The method of making a bulk crystal may comprise making aseed crystal from group III nitride formed using HVPE, MBE, or MOVPE,shaping this group III nitride to have slow-growing sidewalls and afaster-growing face, and using the seed crystal in a bulk growth processsuch as ammonothermal growth or flux growth to form the bulk crystal.

The invention also provides a new ingot or bulk crystal of group IIInitride. The bulk crystal is expressed as Ga_(x1)Al_(y1)In_(1-x1-y1)N(0≦x1≦1, 0≦x1+y1≦1). The bulk crystal has one or more of the followingtraits in any combination: (a) the bulk crystal has a thickness greaterthan 1 mm without any cracks in the crystal above the seed's sidewalledge (in the area bridging the sidewall and new growth on the sidewallof the seed); (b) the bulk crystal has a thickness greater than 1 mm andhas fewer cracks in the crystal above the original seed's edge than acomparative bulk crystal grown under otherwise identical conditions butgrown using a square seed having the same surface area as the originalseed but having m-plane and a-plane walls.

Further, the invention provides new wafers of group III nitride andsemiconductor and optical devices on those wafers as well as methods ofmaking these.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic drawing of the top view of a bulk crystal of groupIII nitride in a conventional method.

In the figure each number represents the followings:

11. A location of a seed crystal,

11A. A side of a-plane on the seed crystal,

11B. A side of m-plane on the seed crystal,

12. A bulk crystal of group III nitride,

13A. Small cracks above the seed crystal edge

13B. A large crack originating from the small crack above the seedcrystal edge.

FIG. 2 is a schematic drawing of the top view of a bulk crystal of groupIII nitride in this invention.

In the figure each number represents the followings:

21. A location of a seed crystal,

22. A bulk crystal of group III nitride.

DETAILED DESCRIPTION OF THE INVENTION Overview

The bulk crystal of the present invention is typically sliced to producegroup III nitride wafers suitable for fabricating various optoelectronicand electronic devices such as LEDs, LD, transistors, and photodetectorsby known techniques. Many optoelectronic and electronic devices arefabricated with thin films of group III nitride alloys (i.e. alloys ofGaN, AlN and InN). The group III nitride alloys are typically expressedas Ga_(x)Al_(y)In_(1-x-y)N (0≦x≦1, 0≦x+y≦1). Since the group IIImetallic elements (i.e. Al, Ga, In) shows similar chemicalcharacteristics, nitrides of these group III elements makes alloys orsolid solution. In addition, crystal growth nature of these group IIInitrides are quite similar.

Due to limited availability and high cost of single crystallinesubstrates of group III nitride, these devices have been fabricated onso-called heteroepitaxial substrates such as sapphire and siliconcarbide. Since the heteroepitaxial substrates are chemically andphysically different from the group III nitride, the device typicallyhas a high density of dislocations (10¹⁸˜10¹⁰ cm⁻²) generated at theinterface between the heteroepitaxial substrate and the device layer.Such dislocations deteriorate performance and reliability of devices,thus substrates composed of crystalline group III nitride such as GaNand AlN are favorable.

Currently, the majority of commercially available GaN substrates isproduced with HVPE, in which it is difficult to reduce the dislocationdensity to less than 10⁵ cm⁻². Although the dislocation density ofHVPE-GaN substrates is a few orders of magnitude lower than GaN film onheteroepitaxial substrates, the dislocation density is still a feworders of magnitude higher than typical silicon devices in electronics.To achieve higher device performance, lower dislocation density isrequired.

To attain dislocation density less than 10⁵ cm⁻², ammonothermal growth,which utilizes supercritical ammonia, has been developed. Theammonothermal method can produce GaN substrates with dislocation densityless than 10⁵ cm⁻². One advantage of the ammonothermal method is thatbulk crystals having a thickness larger than 1 mm can be grown. Theammonothermal method can also be used to grow crystals having variousdopants such as donors (i.e. electron), acceptors (i.e. hole) ormagnetic dopants. However, it is challenging to obtain a bulk crystalover 1 mm thick without cracking. Although the origin and mechanism forcrack formation are not well known, a possible cause would be stressaccumulation inside the crystal due to a slight mismatch of thermalexpansion coefficient or other physical properties between the seedcrystal and grown crystal. To produce group III nitride substrateswithout cracks, it is necessary to obtain crack-free bulk crystal ofgroup III nitride.

Technical Description of the Invention

In an effort to reduce or eliminate cracking inside the bulk crystal ofgroup III nitride having thickness larger than 1 mm, the currentinvention provides a method of making a bulk crystal of group IIInitride in which a single-crystal seed has (1) exposed basal planes onwhich most growth occurs and (2) exposed sidewalls that grow slowlycompared to the basal planes. The exposed surfaces may optionally have asmall miscut angle (e.g. within approximately +/−5 degrees) from thecrystallographic planes, but the miscut is not sufficiently large tocause rapid growth on the sidewalls. The single-crystal seed may begrown by hydride vapor-phase epitaxy (HVPE), molecular beam epitaxy(MBE), metal organic vapor-phase epitaxy (MOVPE), ammonothermal growth,or other method.

The sidewalls of this seed crystal grow more slowly than the face orbasal plane grows when forming a bulk crystal of group III nitride usingthe seed. All sidewalls may grow at a rate that is less than 20% of therate at which a face grows. All sidewalls instead or additionally maygrow at a rate that is less than 20 μm per day in another instance,while optionally a face essentially parallel to a basal plane may growat a rate that is greater than 20 μm per day.

For instance, the seed crystal may have c-planes (i.e. {0001} planes) asbasal planes and only m-planes (i.e. {10-10} planes) as sidewalls. Theexposed surface optionally has a small miscut angle (withinapproximately +/−5 degrees) from the crystallographic planes. Them-plane sidewalls may grow at a rate that is less than 20% of the rateat which a c-plane face grows. Instead or additionally, the m-planesidewalls may grow at a rate that is less than 20 μm per day, whileoptionally a face essentially parallel to a basal c-plane may grow at arate that is greater than 20 μm per day.

In the conventional ammonothermal growth of bulk group III nitride,free-standing GaN wafers fabricated by HVPE are commonly used as a seedcrystal. The free-standing GaN wafers are typically supplied in a roundshape or a square shape. When a whole piece is used as a seed, the seedshape is either a round shape or a square shape. If smaller pieces arecut from the whole piece, the shape is commonly square or rectangular.This type of seed crystal has c-planes, m-planes, and a-planes (i.e.{11-20} planes) or other semipolar planes (e.g. {11-21} planes).

Through our experiments, we found cracks tend to originate from theboundary of the seed crystal and the grown crystal. In other words,cracks occur above the edges of a certain orientation of the seedcrystal and edges of m-plane sidewalls create fewer or no cracks thanother orientations. FIG. 1 shows an example of such situation. When arectangular piece of free-standing GaN having c-plane basal planes isused as a seed (11 in FIG. 1), the seed typically has exposed m-plane(11B in FIG. 1) and a-plane (11A in FIG. 1) sidewalls. During bulkcrystal growth by the ammonothermal method, the crystal becomeshexagonally shaped due to the difference in growth speed on a-planes andm-planes, eventually exposing only m-plane sidewalls (12 in FIG. 1). Wediscovered that the majority of or all cracks occurs over the boundaryof a-planes (13A in FIG. 1), and the regions over the m-plane boundarywere less likely to have cracks. In addition, some of the small cracksextend over a much greater length within the crystal (13B in FIG. 1).

Based on this discovery, the current invention utilizes a seed crystalwhich exposes only slow-growing sidewalls such as m-plane sidewalls inaddition to fast growing basal planes such as c-oriented basal planes asshown in FIG. 2. With this configuration, the grown crystal (22 in FIG.2) does not expand much toward the lateral direction from the seedcrystal (21 in FIG. 1), and crack generation is greatly reduced.

This could be due to the fact that the cracks propagate along m-planes.Another possible cause is that suppressing lateral growth reduces stressaccumulation in the grown crystal. If the cross section of the growncrystal is observed, the crystal consists of growth domains, a domainover one side of the seed's basal plane, a domain over the other side ofthe seed's basal plane and domains over the sidewalls. These domainshave interfaces together and the interface would experience a stress dueto different growth speed, impurity incorporation, growth directionand/or other chemical and physical characteristics.

To minimize dissimilarity of the domains, the current inventionminimizes the number of different crystallographic planes exposed forcrystal growth. The seed crystal in one instance has only c-planes andm-planes exposed, with c-planes forming the basal planes and m-planesforming the sidewalls. Preferably, two seeds are attached together onGa-polar c-planes to expose only N-polar c-planes and m-planes. Thisway, it is possible to reduce/eliminate cracks in bulk group III nitridecrystal.

Since cracks may occur due to accumulated stress, it is preferable togrow the crystal in a symmetrical arrangement. To make the crystalsymmetrical, a seed crystal of hexagonal shape, triangular shape orrhombus shape having m-plane sidewalls is preferred. The length of thesides are preferably equal with +/−10% error so that the grown crystalbecomes symmetrical shape.

This invention is particularly effective when the seed crystal is grownby HVPE, although the seed crystal can be grown by other methods such asan ammonothermal method, MOVPE, and MBE. Currently, it is quitechallenging to obtain a crack-free seed crystal having dislocationdensity less than 10⁵ cm⁻² using HVPE. This is because group III nitridecrystals do not exist naturally. Bulk growth methods such asammonothermal growth or flux growth require a seed crystal forsingle-crystal ingot growth, however, the seed crystal is usuallyobtained by another method such as HVPE. When an HVPE-prepared seed isused, the grown bulk crystal typically cracks when the bulk crystalthickness exceeds 1 mm. By using a seed which only exposes e.g. c-planesand m-planes, cracks originating from the edge of the seed arereduced/eliminated.

COMPARATIVE EXAMPLE Example 1

Single crystalline GaN seed crystal having a basal plane of c-plane isprepared with HVPE. The thickness of the GaN seed is approximately 430microns. By cleaving (along m-plane) and breaking (along a-plane), theHVPE GaN seed is made square shape exposing m-plane and a-planesidewalls. Then, GaN crystal is grown on the seed crystal byammonothermal method using a high-pressure reactor made of Ni—Crsuperalloy.

The inner room of the high-pressure reactor is divided into lower partand the upper part with baffle plates. Approximately 15 g ofpolycrystalline GaN is used as a nutrient and approximately 3.1 g ofsodium is used as a mineralizer. Mineralizer and the seed crystal areplaced in the lower part of the high-pressure reactor and the nutrientis placed in the upper part of the high-pressure reactor. Then, thehigh-pressure reactor is sealed, pumped to a vacuum and filled withanhydrous liquid ammonia. The volumetric ammonia fill factor isapproximately 53%.

The high-pressure reactor is heated at about 510˜520° C. to allowcrystal growth of GaN on the seed. After sufficient amount of time, theammonia is released and the high-pressure reactor is cooled. Theresultant bulk GaN crystal has a thickness of approximately 5 mm. Thefull-width half maximum (FWHM) of the X-ray 002 peak is less than 150arcsec, showing a good microstructure. The crystal has yellowishtransparent color. The crystal as observed with an optical microscopehas small cracks as shown in FIG. 1, 13A and also has a large crack asshown in FIG. 1, 13B. This result demonstrates that cracks in the bulkGaN crystal tend to originate from the interface between the growncrystal and non-m-plane edge of the seed crystal.

Example 2

Two single crystalline GaN seed crystals having a basal plane of c-planeare prepared with HVPE. By cleaving (along m-plane), the HVPE GaN seedis made hexagonal shape exposing only m-plane sidewalls. The thicknessof the seed crystals is approximately 430 microns. The length of eachside is approximately 1 cm with +/−10% error. Also, the orientation ofthe sidewall is m-plane with unintentional miscut angle within 5degrees. The two seed crystals are attached together on Ga-polar c-planesurfaces to make a piece which only exposes N-polar c-planes and m-planesidewalls. Then, a bulk GaN crystal is grown on the hexagonal seedcrystals by ammonothermal method using a high-pressure reactor made ofNi—Cr superalloy.

Similar to the method in Example 1, a hexagonal shape bulk GaN havingthickness approximately 5 mm is grown. The yellowish transparent crystaldoes not have small cracks and does not have large cracks that originateabove the interface between the seed sidewalls and the grown crystal.The X-ray FWHM is less than 150 arcsec, representing a goodmicrostructure.

Example 3

Single crystalline GaN seed crystal having a basal plane of c-plane isprepared with HVPE. By dicing along m-plane, the HVPE GaN seed is madehexagonal shape exposing only m-plane sidewalls. The thickness of theseed crystal was approximately 430 microns. Since dicing creates roughersidewall than cleavage, the seed crystal was immersed in concentratedphosphoric acid maintained at approximately 120° C. for 30 minutes toremove surface or subsurface damage caused by dicing. The length of eachside is approximately 1 cm with +/−10% error. Also, the orientation ofthe sidewall is m-plane with unintentional miscut angle within 5degrees. Then, a bulk GaN crystal is grown on the hexagonal seed crystalby ammonothermal method using a high-pressure reactor made of Ni—Crsuperalloy.

Similar to the method in Example 1, a hexagonal shape bulk GaN havingthickness approximately 5 mm is grown. The yellowish transparent crystalshows neither of small cracks and large cracks originating the interfacebetween the seed sidewalls and the grown crystal. The X-ray FWHM is lessthan 150 arcsec, representing a good microstructure.

Example 4

A hexagonal shaped, single crystalline GaN seed crystal having a basalplane of c-plane is prepared by slicing a bulk crystal of GaN grown bythe ammonothermal method. The bulk GaN crystal grown by theammonothermal method in this example has hexagonal polyhedron shapehaving m-plane sidewalls, and the crystal is sliced parallel to c-planeto form hexagonal-shaped seeds having only m-plane sidewalls. Thethickness of the seed is approximately 500 microns and the length ofeach side is approximately 1 cm with +/−10% error. Also, the orientationof the sidewall is m-plane with unintentional miscut angle within 5degrees. Then, GaN crystal is grown on the hexagonal seed crystal byammonothermal method using a high-pressure reactor made of Ni—Crsuperalloy.

Similar to the method in Example 1, a hexagonal shape bulk GaN havingthickness approximately 5 mm is grown. The yellowish transparent crystalshows neither small cracks nor large cracks originating above theinterface between the seed sidewalls and the grown crystal. The X-rayFWHM is less than 150 arcsec, representing a good microstructure.

Example 5

Similar to Example 4, a hexagonal shaped, single crystalline GaN seedcrystal having a basal plane of c-plane is prepared by slicing a bulkcrystal of GaN grown by a sodium flux method. The bulk GaN crystal grownby a sodium flux method has hexagonal pyramid shape having {10-11} planesidewalls. Slicing the crystal parallel to c-plane makeshexagonal-shaped seeds having {10-11} plane sidewalls. Then, thesidewall is cut along m-plane with a crystal dicer followed by etchingin hot phosphoric acid as explained in Example 3. The thickness of theseed is approximately 560 microns and the length of each side isapproximately 1 cm within +/−10% error. Also, the orientation of thesidewall is m-plane with unintentional miscut angle within 5 degrees.Then, the hexagonal seed crystal is used to grow an ingot ofsingle-crystal GaN by ammonothermal method using a high-pressure reactormade of Ni—Cr superalloy.

Similar to the method in Example 1, a hexagonal shape bulk GaN havingthickness approximately 5 mm is grown. The yellowish transparent crystalshows neither small cracks nor large cracks originating above theinterface between the seed sidewalls and the grown crystal. The X-rayFWHM is less than 150 arcsec, representing a good microstructure.

Example 6

Wafers are formed from bulk group III nitride of the examples above. Anelectronic device is formed on wafers of each of the examples aboveusing conventional circuit fabrication techniques known to those in thefield of making electronic devices using group III nitride (e.g. by ionimplantation, etching, and placing additional layers of materials on thewafers). Likewise, optical and optoelectronic devices are formed onwafers using conventional fabrication techniques.

Advantages and Improvements

The bulk GaN crystal of this invention contains no small or large cracksoriginating the interface between the sidewall of a seed crystal and thegrown crystal. The obtained crack-free bulk GaN crystals are sliced intowafers. These wafers are used for optical devices such as LEDs and laserdiodes or electronic devices such as high-power transistors. Sincecracks deteriorate performances and reliability of these devicessignificantly, this invention can improve the device performance andreliability.

Possible Modifications

Although the preferred embodiment describes bulk crystals of GaN,similar benefit of this invention can be expected for other group IIInitride alloys of various composition, such as AlN, AlGaN, InN, InGaN,or GaAlInN.

Although the preferred embodiment describes GaN seed crystal havingthickness about 430 microns, 500 microns, 560 microns, similar benefitof this invention can be expected for other thicknesses between 100microns to 2000 microns.

Although the preferred embodiment describes ammonothermal growth,similar benefit of this invention can be expected for other bulk growthmethods such as a flux method or high-pressure, high-temperaturesolution growth. In the flux method, a group III metal and a flux suchas sodium are melted together, and nitrogen then dissolves into themelt. One flux method is disclosed in U.S. Pat. No. 5,868,837. Onesuitable high-pressure, high-temperature solution growth method isdisclosed in U.S. Pat. No. 6,273,948. Each of these patents isincorporated by reference herein.

Although the preferred embodiment describes a seed crystal having awidth from one sidewall to a parallel sidewall of approximately 1 cm,similar benefit of this invention is expected for a larger seed such as1″, 2″, 4″, 6″ and larger.

A bulk crystal as described, as made, or as used in any of thedescription above may have a thickness greater than or equal to: 1 mm, 2mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, for instance.

In view of the foregoing, following are various examples of what isdisclosed that are not limiting of the scope of the invention describedhere:

1. A method of growing a bulk crystal of group III nitride having acomposition of Ga_(x1)Al_(y1)In_(1-x1-yl)N (0≦x1≦1, 0≦x1+y1≦1)comprising,

-   -   (a) preparing at least one seed crystal of        Ga_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1, 0≦x2+y2≦1) having basal        planes of c-orientation with miscut angle less than 5 degrees        and all exposed sidewalls of m-orientation with miscut angle        less than 5 degrees;    -   (b) growing the bulk crystal of group III nitride on at least        one basal plane of the seed crystal without creating cracks        above the edges of the seed crystal.

2. A method according to paragraph 1 wherein the seed crystal has ahexagonal shape.

3. A method according to paragraph 1 or paragraph 2 wherein the bulkcrystal has a thickness greater than 1 mm.

4. A method according to paragraph 3 wherein said thickness is greaterthan a thickness of a comparative bulk crystal grown under identicalconditions but grown on a square seed crystal.

5. A method according to any of paragraphs 1 through 4 wherein all ofthe sidewalls of the seed crystal have a length within +/−10% of oneanother.

6. A method according to any of paragraphs 1 through 5 wherein the seedcrystal is obtained by a vapor phase method and at least one of thesidewalls is obtained by cleavage along m-planes.

7. A method according to any of paragraphs 1 through 5 wherein the seedcrystal is obtained by a vapor phase method and at least one of thesidewalls is obtained by mechanical dicing along m-planes and removal ofmechanical damage.

8. A method according to paragraph 7 wherein the removal of mechanicaldamage is carried out by chemical etching in liquid containingphosphoric acid.

9. A method according to any paragraph above wherein the bulk crystal isgrown in supercritical ammonia.

10. A method according to paragraph 9 wherein the seed crystal has ahexagonal shape.

11. A method according to paragraph 9 or paragraph 10 wherein the seedcrystal is obtained from a bulk crystal of group III nitride grown insupercritical ammonia or a melt of group III metal and at least one ofthe sidewalls is a natural facet formed during the growth insupercritical ammonia or a melt of group III metal.

12. A method according to any of paragraphs 9 through 11 wherein thebulk crystal is grown on nitrogen polar surfaces of first and secondseed crystals having c-plane orientation with miscut angle less than 5degrees, each seed crystal having sides that have a length within +/−10%of one another, and wherein a Ga-polar face of the first seed crystal isattached to a Ga-polar face of the second seed crystal.

13. A method according to any paragraph above wherein the bulk crystalis GaN.

14. A method of growing a bulk crystal of group III nitride having acomposition of Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1)comprising forming Ga_(x1)Al_(y1)in_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1) on abasal face of a seed crystal of Ga_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1,0≦x2+y2≦1) and in a direction along an axis perpendicular to a basalplane of the basal face, wherein the seed crystal has only slow-growingedges exposed for growth in a lateral direction that is perpendicular tosaid axis.

15. A method according to paragraph 14 wherein the slow-growing edgesare prismatic faces of the seed crystal.

16. A method according to paragraph 15 wherein the seed crystal has sixprismatic faces.

17. A method of growing a bulk crystal of group III nitride having acomposition of Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1)comprising forming Ga_(x1)Al_(y1)In_(1-x1-y1)N (023 x1≦1, 0≦x1+y1≦1) ona basal face of a seed crystal of Ga_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1,0≦x2+y2≦1) and in a direction along an axis perpendicular to a basalplane of the basal face, wherein the seed crystal has a first and secondbasal face, wherein the seed crystal has a plurality of edge facesextending between the first basal face and the second basal face, saidplurality of edge faces extending completely around a perimeter of thefirst basal face and the second basal face, and wherein each of theplurality of edge faces are oriented with a second plane, said secondplane being different from the basal plane.

18. A method according to paragraph 17 wherein the second plane ism-plane.

19. A method according to paragraph 17 or paragraph 18 wherein the basalplane is c-plane.

20. A method of growing a bulk crystal of group III nitride having acomposition of Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1)comprising forming Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1)comprising forming Ga_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1, 0≦x2+y2≦1) andin a direction along an axis perpendicular to a basal plane of the basalface, wherein the seed crystal has only slow-growing edges exposed forgrowth in a lateral direction that is perpendicular to said axis.

21. A method according to paragraph 20 wherein the slow-growing edgesare prismatic faces of the seed crystal.

22. A method according to paragraph 21 wherein the seed crystal has sixprismatic faces.

23. A method according to any paragraph above wherein x2=1.

24. A bulk crystal of group III nitride made by a method of anyparagraph above.

25. A bulk crystal of group III nitride having a composition ofGa_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1) and a thickness largerthan 1 millimeter wherein the bulk crystal is grown on at least onebasal plane of a seed crystal having a composition ofGa_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1, 0≦x2+y2≦1) and the bulk crystal hasno cracking above the edges of the seed crystal.

26. A bulk crystal of group III nitride having a composition ofGa_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1) and containing a seedcrystal having a composition of Ga_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1,0≦x2+y2≦1), wherein the bulk crystal has a thickness greater than 1 mmand the bulk crystal has fewer cracks within the bulk crystal above theseed crystal's edge than a comparative bulk crystal grown underotherwise identical conditions but grown using a square seed having thesame surface area as said seed crystal but having m-plane and a-planewalls.

27. A bulk crystal of group III nitride according to paragraph 25 orparagraph 26, wherein the seed crystal has the followingcharacteristics:

-   -   (a) the basal plane of the seed is c-plane oriented with miscut        angle less than 5 degrees;    -   (b) all exposed sidewalls are m-planes with miscut angle less        than 5 degrees.

28. A bulk crystal of group III nitride of paragraph 27 wherein the seedcrystal has a hexagonal shape.

29. A bulk crystal of group III nitride of paragraph 28 wherein all ofthe exposed sides of the seed crystal have a length with +/−10% error ofeach other.

30. A bulk crystal of group III nitride of paragraph 25 through 29wherein the bulk crystal is grown in supercritical ammonia.

31. A bulk crystal of group III nitride of paragraph 30 wherein the bulkcrystal contains two seed crystals of c-plane orientation havingpractically identical shape and size with +/−10% error in length, withboth Ga-polar face attached on each other so that only N-polar c-planesand m-planes are exposed for growth.

32. A bulk crystal of Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1)comprising

-   -   (a) a seed crystal composed of Ga_(x2)Al_(y2)In_(1-x2-y2)N        (0≦x2≦1, 0≦x2+y2≦1), the seed crystal having a first basal face,        a second basal face opposite the first basal face, and a        plurality of edge faces extending between the first basal face        and the second basal face, said plurality of edge faces        extending completely around a perimeter of the first basal face        and the second basal face;

(b) a first layer of Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1, 0≦x1+y1≦1) onthe first basal face of said seed crystal

-   -   (c) a second layer of Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1,        0≦x1+y1≦1) on said edge faces of the seed crystal, said second        layer having a minimum thickness and a maximum thickness    -   (d) and wherein the first layer has a thickness of at least five        times the maximum thickness of said second layer. 33. A bulk        crystal of single-crystal Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1,        0≦x1+y1≦1) having a seed crystal as a portion of the bulk        crystal, wherein the bulk crystal has a thickness greater than 1        mm, and the bulk crystal has no cracks within the bulk crystal        above an edge of the seed crystal.

34. A bulk crystal of group III nitride of paragraph 25 through 33wherein the bulk crystal is GaN.

35. A wafer of group III nitride fabricated from the bulk crystal ofgroup III nitride of any of paragraphs 24-34.

36. An electronic, optical, or optoelectronic device formed using awafer of paragraph 35.

37. A method comprising processing a single-crystal ingot ofGa_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1, 0≦x2+y2≦1) produced byheteroepitaxial deposition of the Ga_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1,0≦x2+y2≦1) on a substrate to form a seed crystal that has a first basalface, a second basal face, and a plurality of edge faces extendingbetween the first basal face and the second basal face, said pluralityof edge faces extending completely around a perimeter of the first basalface and the second basal face, wherein said edge faces individuallyhave a growth rate less than 20 microns per day and said first basalface has a growth rate greater than 20 microns per day.

38. A method according to paragraph 37 wherein the seed crystal isformed using HVPE.

39. A method according to paragraph 37 wherein the seed crystal isformed using MOVPE.

40. A method according to paragraph 37 wherein the seed crystal isformed using MBE.

41. A method according to any of paragraphs 37-40 wherein x2=1.

42. A seed crystal of Ga_(x2)Al_(y2)In_(1-x2-)y2N (0≦x2≦1, 0≦x2+y2≦1) asdescribed in any paragraph above.

What is claimed is:
 1. A method of growing a bulk crystal of group IIInitride having a composition of Ga_(x1)Al_(y1)In_(1-x1-y1)N (0≦x1≦1,0≦x1+y1≦1) comprising, (a) preparing at least one seed crystal ofGa_(x2)Al_(y2)In_(1-x2-y2)N (0≦x2≦1, 0≦x2+y2≦1) having basal planes ofc-orientation with miscut angle less than 5 degree and all exposedsidewalls of m-orientation with miscut angle less than 5 degree; (b)growing the bulk crystal of group III nitride on at least one basalplane of the seed crystal without creating cracks above the edges of theseed crystal.
 2. A method according to claim 1 wherein the seed crystalhas a hexagonal shape.
 3. A method according to claim 2 wherein all ofthe sides of the seed crystal have a length within +/−10% of oneanother.
 4. A method according to claim 1 wherein the seed crystal isobtained by a vapor phase method and at least one of the sidewalls isobtained by cleavage along m-planes.
 5. A method according to claim 1wherein the seed crystal is obtained by a vapor phase method and atleast one of the sidewalls is obtained by mechanical dicing alongm-planes and removal of mechanical damage.
 6. A method according toclaim 5 wherein the removal of mechanical damage is carried out bychemical etching in liquid containing phosphoric acid.
 7. A methodaccording to claim 1 wherein the bulk crystal is grown in supercriticalammonia.
 8. A method according to claim 7 wherein the seed crystal ishexagonal shape.
 9. A method according to claim 8 wherein the all sidesof the seed crystal have practically same length with +/−10% error. 10.A method according to claim 7 wherein the seed crystal is obtained by avapor phase method and at least one of the sidewalls is obtained bycleavage along m-planes.
 11. A method according to claim 7 wherein theseed crystal is obtained by a vapor phase method and at least one of thesidewalls is obtained by mechanical dicing along m-planes and removal ofmechanical damage.
 12. A method according to claim 11 wherein theremoval of mechanical damage is carried out by chemical etching inliquid containing phosphoric acid.
 13. A method according to claim 7wherein the seed crystal is obtained from a bulk crystal of group IIInitride grown in supercritical ammonia or a melt of group III metal andat least one of the sidewalls is a natural facet formed during thegrowth in supercritical ammonia or a melt of group III metal.
 14. Amethod according to claim 7 wherein the bulk crystal is grown onnitrogen polar surfaces of two seed crystals having c-plane orientationwith miscut angle less than 5 degree, having practically identical shapeand size with +/−10% error in length, with both Ga-polar face attachedon each other.
 15. A method according to claim 7 wherein the bulkcrystal is GaN.
 16. A bulk crystal formed by a method according toclaim
 1. 17. A bulk crystal formed by a method according to claim
 7. 18.A bulk crystal formed by a method according to claim
 10. 19. A bulkcrystal formed by a method according to claim
 13. 20. A bulk crystalformed by a method according to claim 14.